Learn Unit 10-s -Block Elements (Alkali and Alkaline Earth Metals) with Free Lessons & Tips

Potassium and cesium are sometimes preferred over lithium in photoelectric cells due to their lower ionization energy. Ionization energy is the energy required to remove an electron from an atom or ion in its gaseous state. Both potassium and cesium have lower ionization energies compared to lithium, making it easier to liberate electrons from their surfaces when exposed to light.

In photoelectric cells, the goal is to efficiently convert light energy into electrical energy by causing the emission of electrons from a material's surface (the photoelectric effect). Materials with lower ionization energies can release electrons more readily when illuminated by photons, leading to a more efficient conversion process.

However, the choice of material also depends on various factors such as cost, stability, and practical considerations in the specific application. While potassium and cesium may have advantages in certain cases, lithium could still be chosen for other applications where its properties are more suitable.

When alkali metals like lithium, sodium, or potassium dissolve in liquid ammonia, they form solutions that exhibit interesting color changes. This phenomenon is due to the formation of solvated electrons, which are free-moving electrons surrounded by a shell of solvent molecules (in this case, ammonia molecules).

Initially, as the alkali metal dissolves, the solution appears blue due to the presence of solvated electrons. These solvated electrons are responsible for the blue coloration. However, as more metal dissolves and the concentration of solvated electrons increases, the color of the solution changes to bronze, gold, or even reddish-brown.

This change in color occurs because as the concentration of solvated electrons increases, they begin to interact with each other, forming dimers and other aggregates. These aggregates absorb light differently, leading to a change in the observed color of the solution. The exact color observed depends on factors such as the concentration of solvated electrons and the specific alkali metal involved.

Overall, the color changes observed in alkali metal solutions in liquid ammonia are due to the formation of solvated electrons and their subsequent interactions, which alter the absorption properties of the solution.